1. Field of Invention
This invention relates to the correction of the astigmatism error created in a CRT employing magnetic deflection of the electron beam, and more particularly to the correction of the astigmatism error through the use of a pair of magnetic correction coils whose excitation current is a function of the horizontal and vertical deflection signals.
2. Description of the Prior Art
The use of magnetic deflection coils in CRT displays is preferred over the use of electrostatic deflection plates due to the lower voltages required to produce deflection of the electron beam and higher quality displays obtainable. The use of magnetic deflection coils, however, suffers from the distinct disadvantage that the magnetic field produced by these coils is non-uniform in intensity throughout the cross-sectional area of the electron beam. The non-uniformity of the magnetic field intensity is also dependent upon the frequency of the horizontal and vertical deflection signals due to the finite bandwidth created by the deflection coil inductance. The limitation of frequency values to those within the bandwidth causes the non-uniformity in magnetic field intensity to remain essentially constant, with its magnitude being predictable from the magnitudes of the horizontal and vertical deflection signals. The non-uniformity of the magnetic field causes an astigmatism error which is the result of allowing the magnitude of the deflection force exerted upon an electron within the beam to be dependent upon the relative position of the electron within the cross-sectional area of the beam. This unequal distribution of deflection forces causes a divergence of the electrons in the beam and results in a change of the focal point of the horizontal and vertical components of the beam, which further results in a visual distortion of the beam spot on the CRT screen. The non-uniformity of the magnetic field is most pronounced for maximum magnetic deflection intensities, which coincides with the maximum deflection of the electron beam from the center of the screen. The characteristics of this distortion is well known to the prior art, and results in a compression of the circular beam spot into an ellipse which is transverse to the direction of deflection of the beam at the maximum deflection points. In addition to the non-uniformity of the magnetic deflection field, the magnitudes of the CRT accelerating potential and the CRT beam current also determine the extent of the astigmatism distortion. Since the accelerating potential controls the velocity of the electrons within the beam, it also determines the period of time in which an electron will remain within the area of the magnetic deflection field and the extent to which the unequal deflection forces will affect the electron travel. The beam current determines the electron density of the beam, such that for high beam currents there is a higher electron density and a subsequent greater probability of beam distortion.
In a stroke-write vector display system used for displaying alpha-numeric information in text format on the face of a CRT screen, the effect of the astigmatism error is particularly acute since distortion of the characters displayed on the periphery of the screen will make them illegible. In addition, stroke-write systems employing penetron displays require operation of the CRT at accelerating potentials as low as 6KV and beam currents up to 600 microamps. This results from the penetron display having two superimposed phosphor coatings on the CRT screen; a green phosphor coating applied directly to the screen with a superimposed red phosphor coating. The red phosphor requires the electron beam to operate at low accelerating potentials to avoid the "blossom" effect caused by the impacting of high velocity electrons. The simultaneous requirement for penetration of the red phosphor coating in order to obtain green luminescence requires that the beam power, which is the product of the beam current and accelerating potential, be high with the result that beam currents up to 600 microamps are required. The stroke-write penetron display, therefore, presents a combination in which the conditions for high astigmatism error are present while the desirability for a lack of astigmatism error is acute.
The existence of the CRT astigmatism error is well known in the prior art, and many systems have been developed for correcting this error. However, these correction systems have been developed for use in CRT displays employing single layer phosphor coatings, such as CRTs used for test instrumentation or the CRTs used in color television displays which use either a multiplicity of single layered colored bars, or a single layer of triad grouped color dots. Some prior methods of correction have either proved unsatisfactory for a stroke-write penetron display (due to the low accelerating potentials, high beam currents, and high speed response requirements of such a display), and others have been unnecessarily complex.